As an exhaust gas purifying catalyst to be arranged in an exhaust system (exhaust gas passage) of a diesel engine or the like, and an oxidation catalyst (Diesel Oxidation Catalyst: DOC) for oxidizing hydrocarbons (HC) and nitrogen monoxide (NO) contained in an exhaust gas is known. A diesel particulate filter (DPF) for collecting particulate matter (PM) contained in the exhaust gas and the like is also known.
In the DPF, when an amount of accumulated PM reaches a predetermined value, unburnt fuel (HC) is supplied to the DOC arranged upstream of the DPF and is oxidized to elevate the temperature of the exhaust gas up to a PM combustion temperature such that the PM is burned and removed. This is so-called forced regeneration. Therefore, when the capability of the DOC to oxidize HC (HC oxidation capability of the DOC) is deteriorated, the burning and removal of the PM accumulated in the DPF by the forced regeneration would become insufficient. A deterioration in the regenerating capability of the DPF may cause a shortened interval between regenerating operations, an increase in the exhaust gas pressure, and the like. As a result, a reduction in the fuel efficiency or the like may occur. It is therefore preferable to diagnose the HC oxidation capability of the DOC on board.
According to a technique disclosed in Patent Literature Document 1, for example, exhaust gas temperature sensors are arranged on both upstream and downstream sides of the DOC, and an amount of heat generated by the oxidation in the DOC is estimated on the basis of a difference between exhaust gas temperatures measured by these exhaust gas temperature sensors to diagnose the HC oxidation capability of the DOC.